SusChEM: Visible Light-Driven Reduction of Carbon Dioxide using Heavy Metal-Free Colloidal Quantum Dots as Sensitizers

利用阳光将二氧化碳(CO2)转化为各种可用作燃料的小分子，是利用丰富的太阳能在白天和夜间使用的一种可持续的方式。这种转化涉及的化学反应是复杂的和多步骤的，通常涉及电子从一个分子到另一个分子的转移。“光催化剂”是一种通过使用光能来驱动化学反应，从而降低这种复杂反应的能垒，从而促进这种复杂反应的物种。在化学系化学催化项目的资助下，西北大学的艾米丽·韦斯博士正在进行基础研究，以识别和优化催化剂设计中用于二氧化碳转化的光催化剂，这些催化剂包括吸附在半导体纳米颗粒表面的分子，这些半导体纳米颗粒由无毒、富含稀土的材料制成。她的研究包括对这些无机-有机杂化络合物的化学分析和激光光谱分析，以监测它们在光能激发后在超快时间尺度上的行为。韦斯博士积极参与指导这个项目的本科生研究人员，参与西北大学普通化学新课程的开发，以增加STEM领域中代表性不足的女性和少数族裔学生的留存，并参与Gateway Science shop等项目，该项目为STEM课程的学生，特别是高中准备较弱的学生提供有组织的学习课程。在化学系化学催化计划的资助下，西北大学的Emily Weiss博士正在确定和优化三元无重金属胶体CuInX2(X=S，Se)量子点(Qds)作为可溶性、多位胶体敏化剂的性能中最重要的热力学和动力学参数，用于光催化还原溶液中的二氧化碳。这项工作阐明了胶体QD敏化剂的特殊和独特性质如何通过解决与这项任务相关的几个基本挑战来提高已知具有二氧化碳还原专一性的分子催化剂的性能的机制，即：(I)合成和表面官能化具有足够的还原能力的QD敏化剂，以向助催化剂提供多个电子；(Ii)控制局部氢离子浓度；(Iii)从敏化剂中快速提取空穴，以防止复合和光氧化降解；以及(Iv)敏化剂和辅助催化剂之间的最大电子耦合，以提供快速电子传递途径。一维和二维核磁共振波谱、电子显微镜、电化学测量以及稳态和时间分辨光学测量可以定量表征QD催化剂对的结合亲和力、QD表面的质子(H+)浓度、催化底物和中间体的吸附程度、基本电子和空穴转移步骤的速率常数，以及空穴清除的效率。这些技术是对产物分布和反应速率的气相色谱和液态色谱、核磁共振和红外光谱表征的补充。除了找到新的途径，利用无毒、富含地球的胶体催化剂从阳光和二氧化碳中生产碳基和氢基燃料的社会影响外，这项工作还具有广泛的影响，因为本科生将参与拟议的研究和Weiss博士正在进行的普通化学课程改革，以增加西北大学STEM领域的女性和代表性不足的少数族裔学生的留存率。

The use of sunlight to convert carbon dioxide (CO2) to various small molecules that can be used as fuels is a sustainable way to exploit abundant solar energy for use both during day and night. The chemical reactions involved in this conversion are complicated and multi-step, and typically involve the transfer of electrons from one molecule to another. A "photocatalyst" is a species that lowers the energy barriers for, and thereby facilitates, such complex reactions by using light energy to drive chemical reactions. With funding from the Chemical Catalysis Program of the Chemistry Division, Dr. Emily Weiss of Northwestern University is conducting fundamental studies to identify and optimize photocatalyts for CO2 conversion in catalyst designs that comprise molecules adsorbed to the surfaces of semiconductor nanoparticles made of non-toxic, earth abundant materials. Her studies involve both chemical analysis of these hybrid inorganic-organic complexes and laser spectroscopy to monitor their behaviors on ultrafast timescales after excitation with light energy. Dr. Weiss is actively involved in the mentoring of undergraduate researchers on this project, in the development of a new curriculum for General Chemistry at Northwestern University to increase retention of women and minority students under-represented in STEM fields, and in programs like the Gateway Science Workshop, which offers structured study sessions for students in STEM courses, particularly those students with weaker high school preparation. With funding from the Chemical Catalysis Program of the Chemistry Division, Dr. Emily Weiss of Northwestern University is identifying and optimizing the most important thermodynamic and kinetic parameters in the performance of ternary heavy metal-free colloidal CuInX2 (X = S, Se) quantum dots (QDs) as soluble, multi-site, colloidal sensitizers for photocatalysis of the reduction of CO2 in solution. This work illuminates the mechanisms by which specific and unique properties of a colloidal QD sensitizer enhance the performance of a molecular catalyst with known specificity for CO2 reduction, by addressing several fundamental challenges associated with this task, namely: (i) synthesis and surface functionalization of a QD sensitizer with enough reducing power to donate multiple electrons to the co-catalyst; (ii) control of the local concentration of hydrogen ions; (iii) fast hole extraction from the sensitizer to inhibit recombination and photo-oxidative degradation; and (iv) maximal electronic coupling between the sensitizer and the co-catalyst to provide pathways for fast electron delivery. One and two dimensional nuclear magnetic resonance (NMR) spectroscopy, electron microscopy, electrochemical measurements and steady-state and time resolved optical measurements allow for quantitative characterization of the binding affinity for the QD-catalyst pair, the proton (H+) concentration on the QD surface, the degree of adsorption of the catalytic substrates and intermediates, the rate constants for elementary electron and hole transfer steps, and the efficacy of hole scavenging. These techniques complement the gas and liquid chromatography, NMR, and infrared spectroscopy characterization of product distributions and reaction rates. In addition to the societal impact of finding new pathways to produce carbon- and hydrogen-based fuels from sunlight and CO2 using a non-toxic, earth-abundant colloidal catalyst, this work has broad impact in that undergraduates will be involved both in the proposed research and in ongoing curriculum reform of General Chemistry by Dr. Weiss, in order to increase retention of women and under-represented minority students in STEM fields at Northwestern University.